Electrical image transmission



y 4, 1939. F. c. P. HENROTEAU ELECTRICAL IMAGE TRANSMISSION F iled Sept.5, 1955 5 EM. mm. Hm H Patented July 4, 1939 Francois Charles PierreHenroteau,

Esneux,

Liege, Belgium, assignor to Radio Corporation of America, New York, N.Y., a corporation of Delaware Application September 3, 1935, Serial No.39,075 In Great Britain September 4, 1934 10 Claims.

This invention relates to electrical image trans mission, which includesthe transmission either by wire or wireless of images of actual animateor still objects or of moving or still pictures or transparencies.

In certain systems of television now in use an image is projected on amultiplicity of mutually insulated photosensitive elements arranged onan insulating plate having a metallic backing, so that the potentials ofthe elements are changed in accordance with the light intensity ofcorresponding areas of the image. The elements are scanned by a beam ofcathode rays with the result that the potential of each element whosepotential has been so changed is successively restored, and the fiow ofelectric energy through a transmitting device is varied in accordancewith the successive potential changes resulting from the restoration.

This method of scanning the elements by a beam of cathode rays involvesthe use of electrons having a very high velocity, usually correspondingto a potential diiference of 1,000 volts or more, and the bombardment ofthe elements by such electrons tends to shorten the life of theapparatus.

According to my invention I obtain an effective scanning of the elementswith electrons having a much lower velocity, corresponding to apotential difierence of certainly not more than 50 volts and almostalways less. This restoring of the initial potential of the elements byelectrons having relatively low velocities is made possible by the factthat I do not use a directed beam, as in the prior apparatus describedabove, but successively restore the initial potential of those elements,whose potentials have been changed by the light of the image, by causingeach such element or small group of such elements to be struck by anemission of electrons from a different point closely adjacent thereto,it being thus unnecessary to have the high Velocities required if theelectrons are to be concentrated in a directed beam.

The specific apparatus which I use to carry out my invention comprises acell containing two adjacent plates of transparent insulating material.On the surface of one plate facing the other are formed a multiplicityof mutually insulated photosensitive elements arranged in strips, and onthe surface of this plate facing away from the other plate are a numberof strips of metal back to back with the strips on the first mentionedside and connected together and to a transmitter. On the surface of thesecond plate facing the first plate is a grid composed of parallelphotosensitive strips running at right angles to the strips on the firstplate and all connected through a resistance to a source of low positivepotential. The image is projected through the second plate onto thephotosensitive elements, while at the same time a line of light isprojected through the first plate onto the second plate across thestrips of the grid and is caused to move along these strips, thearrangement being such that, as soon as one line moves off one end ofthe strips, another line begins its travel along them from the otherend.

Each strip has an extension onto a part of the second plate extendingbeyond the first plate, and a beam of cathode rays is caused to strikeeach extension in succession a number of times during movement of a lineof light along the strips, the number of times that each extension isstruck within this period corresponding preferably to the number ofstrips. Owing to their connection to a source of low positive potential,the strips, though photosensitive, cannot normally emit electrons butwhenever the extension of any strip is struck by the beam, the potentialof that strip is lowered to a point where electrons may be emitted fromthat part of it which is illuminated. In this Way an effective scanningof the elements of the first plate is obtained with electrons whosevelocity is merely that corresponding to the potential differencebetween the strips of the grid on the second plate and the elements onthe first plate.

For a more detailed description of my invention I refer to the attacheddrawing, in which:-

Figure 1 is a diagrammatic section of the apparatus.

Figure 2 is a partial magnified view of the surface of the first platefacing away from the second plate.

Figure 3 is a partial magnified view of the surface of the first platefacing the second plate.

Figure 4 is a partial magnified view of the grid on the second plate.

Figure 5 is a partial magnified view of an alternative arrangement tothat shown in Figure 2.

The apparatus shown in Figure 1 comprises a highly evacuated cell Hidivided substantially into two compartments I l and I2. Mounted in thecell very close and parallel to each other are two plates l3 and M oftransparent insulating material such as mica, the first being containedentirely within the compartment l2 and the second extending into bothcompartments. On the surface of the plate l3 facing the plate M are amultiplicity of that light may shine through them onto the plate' I4. Onthe surface of the latter facing the plate I3 is a grid I 9 composed ofparallel photosensitive strips 20 connected through high resistances 2|and a connection 22 to the positive pole of a battery 23 of low voltage.The strips 20 are at right angles to the strips I5 and IS on the plateI3 and are separated by bare-portionsof the plate I4 so that light maypass through the latter to strike the elements on the plate I3. Thestrips 20, moreover, run along the plate I4 beyond the plate I3 into thecompartment I I where they terminate in extensions 24, which, as shownin Fig. 4, may be of greater width and spacing than the strips, and neednot be of photosensitive material.

Outside the cell ID is a lens 25 arranged to focus an image 26 onthe'photosensitive elements on the plate I3, and outside the cell on theopposite side are a plurality of sources of light 21 in the form oflamps with straight filaments 'arranged on an endless band 28' moved ata suitable speed. A lens 29 is arranged to focus the image of thefilament of a lamp on the grid across the strips 20 and the lamps areplaced at such a distance apart on the endless band that the images oftwo filaments arenever projected on the grid at the same time. In asuitable pocket in the chamber II is arranged a cathode ray gun 30provided with a single pair of deflecting plates 3 I, so that the beamof cathode rays may be caused to pass back and forth across theextensions 24. The walls of the chamber I I have a grounded metalliccoating 32 toact as-a shield between the chamber I2 and the cathode rayap' paratus. To complete the shield there is preferably a metalliccoating on the back of the part of plate I4 extending into the chamber II.'

The apparatus described operates as follows: When the image 26 isprojected on the photosensitive elements on the plate I3, the potentialsof these elements are changed in accordance with the light intensitiesof corresponding areas of the image, the electrons emitted beingattracted to the nearest strip 20 and passing to ground at-33. At thesame time that the image to be trans-- mitted is projected on thephotosensitive elements, the image of the filament of one of the lamps,21 is projected on the grid I9 across the photosensitive strips 20 andis caused to move along the strips by movement of the endless band anextension 24, the potential of the corresponding strip 20 is lowered sothat electrons may be emitted from the illuminated portion of it, the

lower potential of the strip preferably being such,

however, that emission will take place only if the potential: of theadjacent photo-sensitive, element or elements on the platev I3 has beenchanged by the light of the image to be transmitted.

There will thus be an emission successively from each point of thestrips 20 which is closest to an element on the plate I3 whose potentialhas been changed by the light of the image to be transmitted. Suchemissionwill strike the element and restore its initial potential withthe result that, through the capacitative coupling with the metallicstrips I6, there will be a corresponding release of energy to thetransmitting device I8, the flow of electric energy through this devicebeingthus controlled in accordance with the successive potential changesresulting from successive restorations.

Where movingimages are to be transmitted, the total quantity of incidentlight integrated over the elements of the strips I5 is not constant, andthe average potential of the strips I6 therefore varies; such variationis however of low frequency compared with the frequencies involved inthe transmission of the image and may be prevented from'afiecting thetransmitter I8 by means of a filter 34.

In some cases the successive reduction of the potential of the strips 20on the plate I4 may give rise, by reason of the capacitative couplingbetween these strips and the strips I6 on the plate I3, to anapproximately sinusoidal variation of potential at the grid of the firstamplifying valve. This interference may be neutralized by applyinganother sinusoidal variation of equal magnitude and opposite phase.

In order to produce a regular scanning action, it is desirable that thepotentials of the strips 20 should be reduced sufficiently to allowelectrons to be emitted only during the passage of the cathode ray inone direction across the extensions 24. This may be obtained by socontrolling the deflecting plates 3I that the beam in its returnmovement (during which no electronic emission from the strips 20 is tobe allowedflmoves so fast across the extensions 24 that insufficienttime is allowed to-reduce the potentials of the strips 20 to the pointat which an electronic emission might take place.

Since the cathode-ray is not itself used for scanning the picture area,it may be concentrated to intercept the plate I4 in the form of a stripinstead of a spot in the apparatus described. Furthermore, highattraction potentials may be used, since the extensions 24 of the strips2!! are not provided with photosensitive elements which could berapidly.destroyed bytheelectronic bombardment. t

A method of preparing the surfaces of the plate I3 so thatit shall havethe strips-referred to,

may be described by way of example. .Lines are printed oneach side ofthe 'plate'with printing ink by means of an engravers copper plate ofthe kind used for the preparation of half-tone. blocks, it being thuspossible to print four hundred. or more lines. per' linear inch. Theside of the plate which is toface plate I4 is then coated with a thinlayerof silver and the other. side with a layer of platinum, by the-wellknown evaporation process in vacuum, .or by. fsputtering. The coatedplateis then subjected for about thirty seconds to a temperatureofabout, 1200 F. This destroys, thebinding material of the ink so that bycareful brushing with a camels hair brush it can be removed in the formof fine powder takingthe superimposed metal with it and leaving lines ofsilver on one side of the plate and lines of'platinum'on the other side.The plate is then subjected for a. short period to a somewhat highertemperaturaand this. causes the, layer of. silver.

to fuse and separate into silver globules. If the thickness of the layerof silver has been properly chosen it is possible to obtain in this waya coating composed of millions of mutually insulated silver globules persquare inch of the plate. The silver globules are then rendered highlyphotosensitive by oxidation and treatment with caesium in the knownmanner.

The side of the plate l4 facing the plate l3 should first be printed,then coated with platinum and then with silver, with the result thatplatinum lines coated with silver are obtained. These lines may then bemade photosensitive by oxidation and treatment with caesium as mentionedabove.

While I have disclosed specifically the use of light to cause theemission of electrons from the strips 20, such emission might, if theplate l3 were perforated, be caused by means of other radiation such asa beam of cathode rays which, if used, would scan the plate I4 in theusual manner, but I prefer to use light since an emission of electronsof lower velocity is obtained in this manner and the plates may, whenlight is used, be sandwiched between two quartz plates, with the resultthat the durability of the apparatus is increased.

I claim:

1. In a television system wherein is provided a multiplicity of mutuallyinsulated photosensitive elements arranged in a plane together with aplurality of photosensitized areas closely adjacent to thephotosensitive elements, the method of electrical image transmissionwhich comprises the steps of projecting an image on the multiplicity ofmutually insulated photosensitive elements arranged in a plane so thatthe potentials of such elements are changed in accordance with the lightintensities of corresponding areas of the image, electronically reducingfrom a predetermined initial value the potential of the photosensitizedareas sequentially, optically ejecting electrons from thephotosensitized areas, restoring the initial potential of thephotosensitive elements by the optically ejected electrons, and varyingthe flow of electrical energy through a transmitting device inaccordance with the successive potential changes resulting from suchrestoration.

2. Apparatus for the electrical transmission of images which comprises acell, a multiplicity of mutually insulated photosensitive elementsarranged in a plane within said cell, means for projecting an image onsaid elements so that the potentials of such elements are simultaneouslychanged in accordance with the light intensities of corresponding areasof the image, means for reducing from a predetermined initial value thepotential of a plurality of photosensitive surfaces adjacent the saidelements, means for emitting electrons from the photosensitive surfaces,means for restoring the initial potentials by the emitted electrons ofthose elements whose potentials have been changed, and means for varyingthe flow of electrical energy through a transmitting device inaccordance with the successive potential changes resulting from suchrestoration.

3. Apparatus for the electrical transmission of images which comprises acell, a multiplicity of mutually insulated photosensitive elementsarranged in a plane within said cell, means for projecting an image onsaid elements so that the potentials of such elements are simultaneouslychanged in accordance with the light intensities of corresponding areasof the image, means for electronically reducing from a predeterminedinitial value the potential of a plurality of photosensitive surfacesadjacent the said elements, means for optically releasing electrons fromthe photosensitive surfaces, means for restoring the initial potentialby thereleased electrons of those elements whose potentials have beenchanged and means for varying the flow of electric energy through aseparate transmitting device for each of said portions in accordancewith the successive potential changes resulting from such restoration.

4. Apparatus for the electrical transmission or images which comprises acell, a multiplicity of mutually insulated photosensitive elementsarranged in a plane within said cell, means for projecting an image onsaid elements so that the potentials of such elements are simultaneouslychanged in accordance with the light intensities of corresponding areasof the image, means for electronically reducing from a predeterminedinitial value the potential of a plurality of photosensitive surfacesadjacent the said elements, means for optically emitting from thephotosensitive surfaces electrons having velocities corresponding to apotential diflerence between the point of emission and any point alongtheir path of travel of less than 50 volts, means for restoring theinitial potentials by the emitted electrons of those elements whosepotentials have been changed and means for varying the flow ofelectrical energy through a transmitting device in accordance with thesuccessive potential changes resulting from such restoration.

5. Apparatus for the electrical transmission of images which comprises acell, a multiplicity of mutually insulated photosensitive elementsarranged in a plane within said cell, means for projecting an image onsaid elements so that the potentials of such elements are simultaneouslychanged in accordance with the light intensities of corresponding areasof the image, means for successively electronically reducing from apredetermined initial value the potential of a plurality ofphotosensitive surfaces adjacent the said elements, means for opticallyemitting electrons from the photosensitive surfaces, said emittedelectrons having velocities corresponding to a potential differencebetween the point of emission and any point along their path of travelof less than 50 volts, means for restoring by the emitted electrons theinitial potentials of those elements whose potentials have been changedand means for varying the flow of electrical energy through atransmitting device in accordance with the successive potential changesresulting from such restoration.

6. Apparatus for the electrical transmission of images which comprises acell, a multiplicity of mutually insulated photosensitive elementsarranged in a plane within said cell, means for projecting an image onsaid elements so that the potentials thereof are simultaneously changedin accordance with the light intensities of corresponding areas of theimage, a grid adjacent to said elements and composed of parallelphotosensitive strips, means for projecting a line of light on the gridacross the strips, means for moving the line along the strips, means fornormally maintaining all the strips at a positive potential such thatelectrons are not emitted from them under the influence of said light,means for successively and repeatedly reducing the poten- 75 ments whosepotentials have been changed by the light of the image are successivelyrestored by emissions of electrons from different points respectivelyclosely adjacent to said elements, and means for varying the flow ofelectrical energy through a transmitting device in accordance with thesuccessive potential changes resulting from such restoration.

7. Apparatus for the electrical transmission of images which comprises acell, a multiplicity of mutually insulated photosensitive elementsarranged in a plane within said cell, means for projecting an image .onsaid elements so that the potentials thereof are simultaneously changedin accordance with the light intensities of corresponding areas of theimage, a grid adjacent to said elements and composed of parallelphotosensitive strips, means for projecting a line of light on the gridacross the strips, means for moving the line along the strips, means fornormally maintaining all the strips at a positive potential such thatelectrons are not emitted from them under the influence of said light,means for causing a beam of cathode rays to strike each strip insuccession a number of times during movement'of the line of light alongsuch strip, thus repeatedly to reduce the potential of such strip to apoint Where electrons may be emitted from an illuminated part of it sothat the initial potentials of those elements whose potentials have beenchanged by the light of the image are successively restored by emissionsof electrons from different points respectively closely adjacent to saidelements, and means for varyingthe flow of electrical energy through atransmitting device in accordance with the successive potential changesresulting from such restoration.

8. Apparatus for the electrical transmission of images comprising acell, a first and a second plate of transparent insulating materialadjacent to one another Within said cell, a multiplicity of mutuallyinsulated photosensitive elements on the surface of the first platefacing the second plate,

a grid composed of parallel strips of photosensitive material on thesurface of the second plate facing the first plate, means for projectingan image through the second plate onto said photosensitive elements sothat the potentials of such elements are simultaneously changed inaccordance with the light intensities of corresponding areas of theimage, means for projecting a line of light through the first plate ontothe second plate across the strips of the grid, means for moving theline of light along the strips, means for normally maintaining all thestrips at a positive potential such that electrons are not emitted fromthem under the influence of the said light, means for causing a beam ofcathode; rays to strike each strip in succession a number of timesduring movement of the line of light along such strip, thus repeatedlyto reduce the'potential of such strip to a point where electrons may beemitted from an illuminated portion of it, so that the initialpotentials of those elements on the first plate whose potentials havebeen changed by the light of the image are successively restored byemissions of electrons from difierent points on the second platerespectively closely adjacent to said elements, and means for varyingthe flow of electric energy through a transmitting device in accordancewith the successivepotential changes resulting from such restoration.

9. In a television system wherein an optical image is arranged to fallupon a photosensitized area, the method of scanning the photosensitizedarea which comprises separately optically scanning the photosensitizedarea in one direction, and electronically scanning the area in amutually perpendicular direction to resolve the complete scanning into aseries of point scannings.

10. In a television system wherein an optical image is arranged to fallupon a photosensitized area, the method of scanning the photosensitizedarea which comprises separately optically scanning line by line thephotosensitized area in one direction, electronically scanning line byline the area in a mutually perpendicular direction to resolve thecomplete scanning into a series of point scannings, and producing imagesignals from the scannings.

FRANCOIS CHARLES PIERRE HENROTEAU.

